Testing system



Jan. 6, I 1,522,006

H. FLETCHER TESTING SYSTEM Filed Nov. 20, 1922 EFFECTIVE VOLT5 mu-hu w 5- QQHESISTANCZ/ I Patented Jan. 6, 1925. V

UNITED STATES n RvEY FLETCHER; OF-N'EW'YORK, 1v. 2., assrGNoRro-wEsTEnNI gma-eta.

or NEW YORK, N. Y., A CORPORATION. or: NEW;YORK:

Jenny, mco-nronarnn,

TESTING YSTEM Application fiI edwNoVember QO, 1922. serialrm-f601,958.

ToaZZ whom .z't may concern:

Be it; known. that I, HARVEY FLETQHER,

a citizen of, the United. States, residing at NewYork, in the county of New York, State of New York, have invented. certain new and useful Improvements in Testing Systems, of which the following is a full, clear,

In accordancefwith the general features of the invention,'a subscribers loop is tested by supplying a definite amount of alternating current energy over the loop to an impedance whichhas a value that is very large in comparison to the impedance of the loop, and then supplying the same amount of energy over the loop to an impedance which is commensurate with the impedance of the loop. By makingtheproper comparison between the two voltages, which cause the energy to be supplied in the two cases, a measure of the eflicienoy of the loop is ob tained.

In the form of the invention. illustrated, the impedance to which the energy is supplied, takes the form of a transformer in which the primary winding is arranged to have an alternative number of active turns, and the secondary wind'ng is connected to a telephone receiver. The constants of the transformer are such that with the receiver. connected to the secondary wlndlng, the 1mpedance looking into the smaller number of primary turns is equal to the impedance of a standard subscribers set. The impedance looking into the entire primary winding is very high in comparison to the impedance of the loop and hence when energy is sup plied to the receiver with the transformer connected so that the entire primary winding is active, the impedance of subscribers loops met in practice has a negligible effect and hence can be assumedtobezzero, which of a switch is thesam'e as the impedancebfarerbloop?- Referring to the drawings Fig-. 1 shows schematically s seritrs pparatus used-inf loop with the connectedmaking ate'st,

Fig. '2-shows an alternative arrangement-f of the central office Fig. 1.

Fig. 3 shows characteristic curves the relation between thedirect'currentre sistance of a certain type oftarbsnatraas apparatus shown in mitter'," and tl1"e alternating voltage gen erated by the transmitter when "it subjected to various degrees ofagitation.

Tn order to make a test two men re-re; qulred, one to makethe circuit changejand i" adjustment at the central it oflice, Y and the other to make them at the subscribers station. Resistance 1, whichflh'as a value'equal to the resistanceof a subscriberstransmitter when subjected to'standard agitation, is substituted for the 'subscribers-' transmitter: Resistance 2 is then adjusted until 'r'neterfAg' shows that adirect .75 current v of the desired battery 3 through Winding 4, line c'onductor 5, resistance 1, choke'coi'l 12, line conductor 6, and winding 7.: Condensers 50, 51,"'

52 and 53 are provided to eXcludethef-direc't current from undesired portions of the cir cuit and-hence it is confined tothepa'th justtraced; Resistance 1 is transmitter T. If

then replaced by; desired, the substitution of resistance 1 for the 'transmittenTjand vice versa,

terminal 21, a resistance 9 is adjusted until the alternating current voltmeter V shows that voltage of a desired standard value, e. g., 10 volts, is being supplied to the line by generator 10, which'preferably is a band frequency generator that delivers current 7 may be accomplished by means 8. 'With switch 20 connected toloop and energizes receiver R, which is} placed in bridge of the line by means of the transformer'which comprises primary wind ing 13and secondary winding 14. With swtohQO connected to terminal 21 the im-- pedance of the transformer looking into Winding. l3' -isivery high: compared to the impedance of the loop and hence a definite voltage, as indicated by voltmeter V, will cause receiver R to give, for practical purposes, the same sound output regardless of the length of the subscribers loops which are met in practice. in other words, the condition is such that the amount of current being supplied over the loop is so small that the voltage drop, between the central oflice and the subscribers station is negligible and hence the effect is the same as if the loop had zero impedance. Choke coil 12 keeps the above mentioned alternating current from flowing through transmitter T and condenser 18. The receiver which is now giving a definite sound output is acoustically coupled to the transmitter thus subjecting the transmitter to an amount of agitation which is a function of the energy which is being supplied over the loop to the receiver. If desired, a tube 15 may be used to inclose an air column between the transmitter and the receiver.

The agitation of the transmitter causes an increase in its resistance and hence a corresponding decrease in the deflection of meter A, which measures the direct current supplied to the transmitter by battery 3. The change in the deflection of meter A, is a function of the increase in the resistance of transmitter T, which, in turn, is a function of the alternating voltage generated by the transmitter, as is shown by the curves in Fig. 3. For the example chosen, curve 6 is the one of interest, as it shows the relation between the alternating voltage gen erated by the transmitter and the effective direct current resistance of the transmitter for the case where the steady state current through the transmitter is milliamperes. The alternating voltage generated by the transmitter is a function of the output of receiver R- and hence the change in deflection of meter A, is a measure of the output of receiver R which, in turn, is a measure of the alternating current energy being supplied over the transmission line.

Switch 20 is now transferred from terminal 21 to terminal 22. If resistance 9 is adjusted until meter A, indicates the same change in deflection, upon agitation of the transmitter T by receiver R as it did for the case where switch 20 was connected to terminal 21, it is apparent that the output of receiver R is the same as it was when switch 20 was connected to 21. It is apparent, therefore, that the same amount of alternating current energy was supplied over the line for the two positions of switch 20.

Meter A, must have a relatively high range, which for the example chosen would mean a full scale deflection of at least 90 milliamperes. The decrease of the direct current caused by the agitation of the transmitter may be so small that it cannot be sufiiciently accurate on the meter of such high range. To obtain more accurate readings, a second meter A of lower full scale deflection, e. g., 15 milliamperes, may be inserted in series with meter A, and a battery 16 in series with resistance 17, used to set meter A back so that it will be operable to indicate changes in a current of the order of 90 milliamperes. The change in deflection of meter A, upon agitation of the transmitter may therefore'be used as a measure of the output of receiver R.

The reading of voltmeter V, is noted after the adjustments have been made with switch 20 connected to terminal 22 and by making the proper comparison between this value of voltage and the value of voltage for the case where switch 20 was connected to terminal 21, a measure of the transmission efiiciency of the subscribers loop can be obtained. This may be seen from the following mathematical calculation.

Let: Z, be the impedance looking in the taps 21 and 23 of the transformer with the receiver of impedance R connected to the secondary winding 14.

Z, be the corresponding impedance lookingin the taps 22 and 23.

M, be the mutual impedance between entire winding 13 and the winding 14.

M be the mutual impedance between the section 22-23 of winding 13, and wind ing 14.

P, be the self impedance of entire winding 13 with winding 14 open. I

1? be the self impedance section 2223 of winding 13 with winding 14 open.

S be the self impedance of the secondary winding 14 with primary winding open.

The current inv the receiver for the two connections 18 given by I M1 E1 (3) 1 S+R Z sinh XL+Z, cosh XL and I 2 M E, W 2 S+R Z sinh XL+ Z cosh XL here: 7

I, is the current through the receiver for the case where the entire primary winding lUO V for'the case where the entire primary winding is active.

E5, is the voltage indicated by voltmeter V for the case where only section 2223 is active.

Z is the surge impedance ofthe subscribers loop.

X isthe propagation constant of the. sub scribers loop.

L is the'length of the loop.

"When'a'zer o loop is used, L20 and equations (3) and'(4') become M E10 ein-z: and

I20=S+R'T20 Where the addition of the subscript O to a character indicates that the character represents the quantityfor the case where a zero loop is used,

Since resistance 9 is respectively adjusted so that receiver R has the same output when switch 20 is connected to terminal 21, and to terminal 22, it is apparent that the current passingthrough thereceiveris the same in theatwo cases,or in other words 1 :1 also I :1 v

v lBy iiiaking the adjustment so that 11 :1? we. have the condition that,

Efficiency log It is seen that equation (13) gives the efiiciency of the loop in terms, of a constant C and E and-E which are the two voltages measured on any given loop for the two positions of switch 20. Iftransforiner 30 were ideal C would be the ratio of the number of primary turns for the two positions of switch 20. For the actual transformer it can be determined from a test on a zero loop for which log C=log :0

In the not unusual case where the surge impedanceot the loop is equal to the 1mpedance oi. a standard subset, l. e., Z:Z equation (9) becomes and hence M s I 2 (7) (from equation 6) Dividing equation (4:) ,by equation (7) 1 EZZZ 8 E, (Zsinh XL+Z cosh xL (a "E, 2 811111 xilcosh XL This is the ratio of the voltage at the substation end of the line to the voltage at the central oflice end of the line when the definite amountaof energy is being delivered to an impedance which-is equal to the impedance of a standard subset, and hence is a measure of the eiiiciency of the loop. If desired the efliciency may be expressed as a Since 11 :11 we can equate the right-hand Efficiency log members of equations (5) and (6) and solve for E whence in Fig. 1 except that there are inserted impedances 31 having values such that the impedance looking into the repeating coil from the loop side is equal to that obtained when an average telephone connection is made. The voltmeter for measuring voltage ap plied to the line is then placed as indicated at V instead of as indicated at V inFig. 1; With this arrangement the measurements are made in exactly the same manner as previously described. In this case the equations which were obtained in deriving the efficiency of the loop contain a term for impedances 31, but the final expression for the loop eflicienoy isthe same, viz,

Efficiency log log C In this case the constant C is equal to tained in the manner previously described by a test on a zero loop 1n which case In the specification, mention has been made of values of current and voltage, but it is to be understood that these values are merely illustrative, and that values of voltage, current, and impedance should be chosen which are the most suitable for the apparatus involved.

The invention claimed is:

1. In a system for measuring the efiiciency of a transmission line, a transmission line, atransmitter in bridge of said line, a receiver connected in bridge of said line by means of a transformer which has an alternate number of turns, means for sending direct current over the line to said transmitter, means for sending alternating current over the line to energize said receiver, and means for measuring the voltage which causes said alternating current to flow.

2. In a system for measuring the eiiiciency t a transmission line, a transmission line, a transmitter and a receiver in bridge of said line, means for sending direct current over the line to said transmitter, means for sending alternating current over the line to energize said receiver, means for varying the impedance which aid receiver otlers to said alternating current, and means for substantially excluding said alternating current from said transmitter.

In a system for measuring the efliciency of a transmission line, a transmission line, a transmitter and a receiver in bridge of said line, means for sending direct current over the line to said transmitter, means for sending alternating current over the line to energize said receiver, means for varying the impedance which said receiver otters to said alternating current, said alternating current having energy frequency spectrum corresponding approximately to voicecurrent, and means for measuring the voltage which causes said alternating current, to flow 4. In a system for'measuring the efficiency of a transmission line, a transmission line, a transmitter and a receiver in bridge oi said line, means for sending'direct current over the line to said transmitter, means for sending alternating current over the line to energize said receiver, means "for varying the impedance which said receiver ofiers to said alternating current, means for measuring the voltage which causes said alternating current to flow, and means responsive to changes in the direct current through said transmitter.

5. In a system for measuring the etiiciency of a transmission line, a transmission line, a transmitter and a receiver in bridge of said transmission line, means for sending direct current over the line to said transmitter, means for sending alternating current over the line to energize said receiver, means "for varying the impedance which said receiver offers to said alternating current, means for measuring the voltage which causes said alternating current to flow, means for substantially excluding said alternating current trom said transmitter, and means; responsive to changes in the direct current through said transmitter.

6. A method of determining the efliciency of a telephone loop which comprises delivering a definite amount of energy over the loop to an impedance which is large relative to the impedance of the loop, delivering a definite amount of energy over the loop to an impedance which is approximately equal to the impedance of a telephone subset, and comparing the two voltages which cause said amounts of energy to be delivered.

'7. A method of determining the efiiciency of a transmission line which comprises delivering a definite amount of energy over said line to a telephone receiver which is connected in bridge of the line so as to offer an impedance which is large in comparison to the impedance of the line, delivering a definite amount of energy over said line to said telephone receiver connected in bridge of the line so as to otter an impedance which is commensurate with the impedance of the line, and comparing the two voltages which cause said amounts of energy to be delivered.

In witness whereof, I hereunto subscribe my name this 17th day of November. A. D.

HARVEY FLETCHER.

llO 

